1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Description of the Background Art
In electrophotographic image forming apparatus, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical, looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image under heat and pressure.
One conventional type of fuser assembly employed in the fixing device is an endless belt looped for rotation around a generally cylindrical, stationary metal pipe, typically formed by bending a thin sheet of conductive metal into a rolled configuration, which has its outer circumference entirely or partially facing the inner surface of the looped fuser belt. The metal pipe has its circumference subjected to heating, for example, by radiation, from which heat is radially transferred to the length of the fuser belt rotating around the metal pipe.
Using the combination of a looped belt and a thin-walled metal pipe, the fuser assembly allows for heating the fixing nip swiftly and uniformly, resulting in shorter periods of warm-up time and first-print time required to complete an initial print job upon startup. This type of fixing device therefore has high immunity against printing failures caused by insufficient heating of the fixing nip in high-speed applications.
One problem encountered in a fuser assembly formed of a combination of cylindrical members (e.g., an endless looped belt and a metal pipe) is its vulnerability to wear where the cylindrical members, one rotatable around the other held substantially stationary, come into repeated frictional contact with each other so that the contacting surfaces chafe and abrade during operation. To prevent such wear, a common practice is to provide a lubricant to reduce frictional resistance between the two contacting surfaces.
For example, one conventional fixing device employs a cylindrical fixing sleeve which is subjected to induction heating while rotatably held around a cylindrical fixing roller to form a fuser assembly. The fixing roller has an outer circumference thereof formed of foamed material impregnated with a lubricant. The lubricant thus provided between the fixing sleeve and the fixing roller, the former rotatable around the latter, serves to reduce friction between the cylindrical surfaces during operation.
Although generally successful for its intended purposes, the provision of lubricant does not have a durable, long-lasting effect when applied to the fuser assembly formed of an endless looped belt and a metal pipe described above. This is because the lubricant tends to be squeezed out or flow away from the area of contact over the circumferential surface of the metal pipe. Such displacement of lubricant results in a loss of lubrication and high frictional resistance at the belt/pipe interface, which, over time, leads to damage and other concomitant failures of the fixing device.